Non-Hermitian degeneracies of internal-external mode pairs in dielectric microdisks

Open quantum and wave systems can exhibit non-Hermitian degeneracies called exceptional points, where both the eigenvalues and the corresponding eigenstates coalesce. Previously, such exceptional points have been investigated in dielectric microcavities in terms of optical modes which are well confined inside the cavity. However, beside these so-called "internal modes" with a relatively high quality factor, there exists another kind of mode called "external modes," which have a large decay rate and almost zero intensity inside the cavity. In the present paper, we demonstrate the physical significance of the external modes via the occurrence of exceptional points of internal-external mode pairs for transverse electric polarization. Our numerical studies show that these exceptional points can be achieved by either a boundary deformation of the microdisk or by introducing absorption into a circular cavity. (C) 2019 Chinese Laser Press